1. Field of the Invention
The present invention generally relates to Josephson devices, methods of forming Josephson devices and superconductor circuits, and more particularly to a Josephson device that uses an oxide superconductor, a method of forming such a Josephson device, and a superconductor circuit having such a Josephson device.
2. Description of the Related Art
Ever since the YBa2Cu3Oy (YBCO) copper oxide superconductor having a critical temperature of 92 K was found in 1987, active research has been made on oxide superconductors. The Josephson device using the oxide superconductor can obtain a high IcRn product and a junction that has a high superconductive transition temperature. The IcRn product is a product of a maximum superconducting current value (critical current value) Ic that can flow through the superconducting junction at a certain temperature, and a resistance value Rn that is obtained when the superconducting state breaks down and the superconducting junction assumes the normal conducting (or non-superconducting) state. This IcRn product is an index that qualitatively represents the magnitude of the signal and the magnitude of the sensitivity at the time of the switching. By use of such a Josephson device, it is possible to realize a superconductor circuit having a high-speed operation and a sensor having a high sensitivity at temperatures of 40 K or higher.
The technique for forming or fabricating the Josephson device utilizing the Josephson effect is essential in forming devices using the oxide superconductor. Various structures, such as the bi-crystal type, micro-bridge type, step-edge type and ramp-edge type, have been proposed for the Josephson device using the oxide superconductor.
The bi-crystal type junction can be formed with ease by depositing a superconductor film on a substrate which is obtained by bonding surfaces having different crystal orientations, because crystal grains that act as a barrier layer within the superconductor film are formed on the substrate interface. Devices using this bi-crystal type junction, such as the SQUID devices, have already been developed to the stage of merchandising. However, the bi-crystal type junction can only be formed at the bonding interface of the substrate, and is unsuited for application to high-performance devices that require complex structures.
On the other hand, in the case of the stacked-type Josephson device or the ramp-edge type Josephson device shown in FIG. 1A, there are no restrictions on the location where the junction is to be formed, and such devices are advantageous when realizing large-scale integration of the superconductor circuits. The ramp-edge type Josephson device shown in FIG. 1A has a substrate 101, and layers 102, 103 and 104 that are stacked on the substrate 101. The lower electrode layer 102 is made of an oxide superconductor. The insulator layer 103 is interposed between the lower electrode layer 102 and the upper electrode layer 104. A junction 105 is formed via a barrier layer 102a that is formed on a sloping surface of the lower electrode layer 102 between the upper and lower electrode layers 104 and 102. The barrier layer of the stacked-type or ramp-edge type Josephson device is made of a non-superconducting material such as PrBa2Cu3Oy, SrTiO3, PrGaO3 and CeO2 or, formed by an Interface-Engineered Junction (IEJ). The ramp-edge type Josephson device having the IEJ shown in FIG. 1B has a substrate 101, and layers 102 and 103 that are stacked on the substrate 101. In FIG. 1B, those parts that are the same as those corresponding parts in FIG. 1A are designated by the same reference numerals, and a description thereof will be omitted. The barrier layer 102a is formed to a thickness of several nm on the sloping surface of the lower electrode 102, by bombarding Ar ions on the sloping surface so as to make the barrier layer 102a amorphous and forming the upper electrode layer 103 (not shown in FIG. 1B) so as to cover the barrier layer 102a. For this reason, compared to the Josephson device having the junction made of a conductor material, the Josephson device having the IEJ can suppress inconsistencies in the junction characteristics and realize a relatively high IcRn product.
With respect to the Josephson device having the IEJ function, T. Satoh et al., “High-Temperature Superconducting Edge-Type Josephson Junctions with Modified Interfaces”, IEEE Trans. Appl. Supercond. Vol. 9, No. 2, June 1999, pp. 3141-3144 reported experimental results of 10 junctions at a temperature of 4.2 K including an IcRn product of 1.5 mV to 3 mV and a 1σ (σ denotes standard deviation) of the maximum current value Ic of approximately 8%. In addition, Y. Soutome et al., “HTS Surface-Modified Junctions with Integrated Ground-Planes for SFQ Circuits”, IEICE Trans. Electron. Vol. E85-C, No. 3, March 2002, pp. 759-763 reported experimental results of 100 junctions at a temperature of 4.2 K including an IcRn product of 1.5 mV and a 1σ (σ denotes standard deviation) of the maximum superconducting current value Ic of approximately 7.9%.
When considering the application of the Josephson junction to the devices, it is desirable from the point of view of reducing the system cost to realize a Josephson device that can operate at a high speed at a high temperature and has a high sensitivity. The IcRn product must be large in order to realize such a Josephson device, but a sufficiently large IcRn product has not be obtained in T. Satoh et al. and Y. Soutome et al. referred above.